As is known in the art, conventional maritime radars for shipboard navigation and for coastal surveillance (e.g., land-based Vessel Tracking Systems (VTS)) detect and measure the position of ships and other discrete contacts in two-dimensional space. This space is typically idealized as a local-level (tangent) plane to the Earth coordinatized in natural radar polar coordinates of range R and true azimuth A or in cartesian coordinates (x, y) with positive x pointing in an Easterly direction and positive y pointing North. When these radars are also configured with Automatic Radar Plotting Aids (ARPA), the position of a contact is tracked over time, and a velocity vector is derived.
Shipboard ARPA radars are used to automatically generate alerts for potential collisions by automatically detecting and tracking other ships. Alerts can be generated under certain conditions involving the relative motion of ownership and the contact under track by the radar.
As is also known in the art, conventional shipboard radars have been used by law enforcement entities, such as the United States Coast Guard, to monitor national waters for natural resource utilization (fishing), smuggling (drugs), and illegal immigration. A growing threat to commercial and military shipping stems from use of small boats to damage large ships or other maritime infrastructure, such as oil platforms.
Standard shipboard radars are equipped with an automatic target detection and tracking feature that meets the specifications of the International Electrotechnical Commission (IEC) for Automatic Radar Plotting Aids (ARPA). The ARPA standard was written in support of ship collision avoidance and its tracking performance requirements suit that application well. However, it has been shown that a cluster or “swarm” of small, maneuverable boats can “confuse” the ARPA tracker making it difficult to assess the movement and the number of boats of interest.
For example, an object of interest may attempt to “get lost” in a cluster of boats, often fishing boats, that happen to be in a small area. While the object of interest may be tracked into the fishing boat cluster, it is often difficult to maintain individual track on it while within the cluster. By weaving in and out close to the relatively stationary fishing boats, the object of interest can likely confuse the tracker processing. Once emerging from the cluster, the object of interest will be given a ‘head start’, on the order of about thirty seconds, for example, while the ARPA tracker tries to re-acquire an individual track, assuming that the contact is within an automatic acquisition zone. By using the clutter provided by the fishing boat, plus the track re-acquisition time, a high speed boat can advance significantly toward its goal without providing a clear track on which to base situational awareness.